The present invention relates to refrigerated display cabinets or cases having at least one air conduit and an ambient air defrost system. Of primary concern are display cases having access openings in their front walls. Both within the specification and the claims of the present application, all references to refrigeration apparatus or refrigeration operations are intended to include cooling both at a temperature below 32.degree. F., such as associated with frozen food display cases, and in excess of 32.degree. F., such as typically associated with dairy food and fresh meat display cases.
Refrigerated display cases having only a single air conduit with both front or top access openings have been used for many years. Such open front cases are conventionally utilized for displaying dairy and meat products.
In the operation of all types of refrigerated display cases, it is desirable to include a system capable of automatically defrosting the display case. The defrost cycle can be actuated either at set periodic times or when the frost buildup within the system has reached a certain predetermined level. Such systems are typically thermostatically controlled so as to switch from a refrigeration cycle to a defrost cycle of operation. By this manner of operation, it is possible to avoid any significant frost buildup within the display case.
Typically within the prior art, there have been three different approaches employed for defrosting refrigerated display cases. The first approach involves the use of electric resistance heaters which are arranged adjacent to the refrigerated coils of the refrigeration mechanism. During a defrost cycle, these heaters supply heat in an effort to melt the frost buildup on the coils but also add warmer air to the air conduit for circulation within the case. The particular technique is relatively simple both in its construction and operation. However, since the electrical heaters are high voltage heaters that utilize significant electricity during operation, the rapidly increasing cost of electricity makes it extremely uneconomical to employ such systems. Furthermore, the warm air circulated in the case can raise the temperature of the case too high. Thus, attempts have been made to find other alternatives to such a system.
A second type of system circulates hot compressed gaseous refrigerant through the refrigeration coils during the defrost cycle. During the defrost cycle, a valve control mechanism shuts off the supply of refrigerant to the refrigeration coils and alternatively feeds superheated compressed gaseous refrigerant through the coils. This hot gas serves to melt any frost that has formed on the refrigeration coils but simultaneously provides heat within the air conduit which can be circulated through the display case, which again is disadvantageous. While this type of system does not suffer from the high cost of operation of the electrical heater defrost system, the heated gas system involves a relatively high construction cost. Due to the requirement that the system be able to selectively switch between the supplying of heated gas and the supplying of refrigerant to the refrigeration coils, a complicated valving structure must be provided. Such a mechanism significantly increases the cost of construction of the display case. In addition, the provision of such a complicated system only increases the number of complex parts capable of breaking down and requiring costly repairs.
The third type of system employed for defrosting display cabinets relies upon the circulation of ambient air. It is this general category with which the invention of the present application is concerned. In such air defrost systems ambient air is often propelled through the same air conduits which are used to contain the refrigerated air during a refrigeration cycle of operation. Such air circulation encounters the problem that warm ambient air can be dumped into the product display space and the stored products can begin to thaw. In order to avoid such thermal shock, it is desirable to arrange for the circulation of ambient air into contact with the refrigeration coils while avoiding the dumping of warm ambient air into the product display space.
In open front refrigerated display cabinets having a plurality of air conduits for the circulation of multiple air bands, the circulation of the defrost air in the top most portion of the inner air conduit should be avoided since air flowing through the outlet port of the inner air conduit flows directly into the product display space. It is considered necessary, however, to circulate the ambient air through a portion of the inner air conduit so that this air which has a high specific heat can be used to defrost the refrigeration coils.
Another desirable feature in display cabinets having air defrost systems is to utilize air moving means which circulate both the refrigeration air band and the ambient air band during the defrost cycle of operation in the same direction whereby unidirectional fan motors can be employed. Another desirable construction feature is to design the refrigerated display cabinet with air conduits which extend the full length of the cabinet so that internal fixed baffles and partitioning wall structures can be avoided since such construction details materially raise manufacturing costs.
It is therefore, desirable to avoid the above problems while providing for an air defrost system in refrigerated display cabinets by providing a cabinet structure which permits the circulation of ambient air during the defrost cycle in the same direction as the refrigerated air band in the inner air conduit in a manner which eliminates the circulation of ambient air in the upper portion of this inner air conduit. At the same time, the present construction permits the circulation of both the refrigeration air bands and the ambient defrost air bands with the same motor-driven fan systems.
One type of system which employs ambient air during the defrost cycle is exemplified by those embodiments illustrated in U.S. Pat. Nos. 3,403,525, 3,850,003 and 3,937,033, all to Beckwith et al. Each of these systems uses fans separate from the main air circulating fans and cooperating structures which increases original construction cost. These extra fans are turned on during the defrost cycle for pulling ambient air from outside of the display case into the air conduits. A second type of system is illustrated in U.S. Pat. No. 3,082,612 to Beckwith, which system draws ambient air into the main circulation path through ports located in the lower front panel of the refrigerated display case. Such ports are normally closed during the refrigeration cycle and are opened during the defrost cycle. The Beckwith et al U.S. Pat. Nos. 3,850,003 and 3,937,033 both indicate that the concepts described in U.S. Pat. Nos. 3,082,612 and 3,403,525 did not prove to be practical and hence were not commercially feasible.
The Beckwith et al U.S. Pat. Nos. 3,850,003 and 3,937,033 show air defrost systems in which the air curtain across the access opening of the cabinet is interrupted during the defrost cycle which then causes refrigerated air contained within the display space to flow into the upper portion of the inner air conduits. This evacuation of the refrigerated air causes an inflow into the display space of ambient air due to the partial vacuum created. The net effect is to cause thermal shock to the stored products in a manner which should be avoided.
U.S. Pat. No. 3,403,525 to Beckwith et al also shows a cabinet which has a defrost and air defrost system which permits the flow of ambient air in the upper portion of the inner air conduit whereby thermal shock to the stored products can occur. Another feature of this refrigerated display cabinet is that the various air conduits are not continuous along the longitudinal length of the case whereby construction costs are considerably increased.
A third type of ambient air defrosting system is shown in U.S. Pat. No. 4,144,720 to Subera et al, which is assigned to the same assignee as the present application. In this patent, an open front refrigerated display case having primary and secondary air conduits is disclosed. In this sytem, reversible fans can be employed for reversing the direction of flow of air within the conduits and simultaneously drawing in air from outside of the display case.
Another system employing reversible fans for ambient air defrost is shown in U.S. Pat. No. 4,026,121 to Aokage. This patent, however, refers to short-circuiting the air flow between the primary and secondary air bands for the purpose of supplying warmer air to the primary band.
Other U.S. Pat. Nos. which show the use of various slats and gates to control air flow are: 2,124,268 to Williams; 2,525,868 and 2,525,869 both to Corhanidis; 3,094,851 and 3,122,892 both to Beckwith et al; 3,115,017 to Kocher et al; 3,226,945 to Spencer; 4,072,488 to Johnston; 4,148,197 to Karashima; 4,122,687 to McKee; and 3,319,557 to Perez.
U.S. Pat. No. 3,444,698 to Lorenz shows a refrigeration unit in which the fan speed is varied in the refrigeration cycle in order to more closely maintain a constant rate of heat transfer from the air to the heat exchanger.
One embodiment of an open front single air conduit display case using an ambient air defrost system is shown in published British patent application GB No. 2,016,669. This British application indicates that it corresponds to U.S. patent application Ser. No. 884,697 filed Mar. 8, 1978.
It has been recognized that an ambient air defrost operation can be incorporated into an open top refrigerated display case as disclosed in U.S. Pat. No. 4,120,174 to Johnston. The Johnston patent illustrates an open top case having a single air conduit extending around the case. During the refrigeration cycle, the air flows in a first direction and during the defrost cycle the direction of the air flow is reversed with ambient air being drawn into the conduit. The quantity of air flow during the defrost cycle is greater than during refrigeration. The defrost air, after passing through the conduit, is expelled in a direction up and over the refrigerated case.
Similar to the open top display case of the above-identified patent to Johnston, there also are open front single air conduit cases which employ the same air defrost techniques as disclosed by the Johnston patent. During the defrost operation of such air defrost cases, the volume of the reverse air flow is relatively high and significantly greater than the air flow during the refrigeration cycle. Such a greater flow is utilized so as to minimize the time needed for the defrost operation. The higher air volume flow in such cases causes the warmer defrost air to reenter the display case which potentially can damage the products in the case. In addition, the greater air flow causing the cold air to flow upwardly can result in hitting the customer in the face with such cold air.
In the above-described refrigerated display cases, the protective air curtain across the access opening is typically eliminated during the defrost cycle of operation. The elimination of the air curtain is particularly common in the prior art cases with ambient air defrost systems. Under some conditions, however, it may be undersirable for this protective air curtain to be eliminated even during the defrost cycle.